check-vulkan's Vulkan shaders not included in relocatable pressure-vessel tarball

Minor issue spotted in https://github.com/ValveSoftware/steam-runtime/issues/312:

libexec/steam-runtime-tools-0/shaders is missing from the relocatable pressure-vessel tarball, so check-vulkan cannot test actually drawing something when we ask a user to run the newer s-r-s-i bundled with pressure-vessel, like I did in https://github.com/ValveSoftware/steam-runtime/issues/312#issuecomment-788834149.

Short-term, we should add that directory. This should be a trivial change.

Longer-term, since check-vulkan uses GLib already, we should maybe use the GResource API to embed the shaders in the executable itself. GResource was new in GLib 2.32, so it should be available even in scout - but if the version available in 2.32 is not mature enough, or if our backport of Meson has trouble with it, then we should not put a lot of time or effort into doing this.